1. Field of the Invention
The present invention relates to a high-intensity-discharge (HID) lamp in which a metal halide continuously emitting light by molecular radiation is sealed within a light transmitting bulb and light is produced by arc discharge, thereby achieving outstanding color rendering properties and high efficacy.
2. Related Art of the Invention
In recent years, HID lamps, and in particular, metal halide lamps, have been replacing halogen lamps as high-output point light sources in various applications including stage and television lighting and liquid-crystal video projector light sources because of their high efficacy and excellent color rendering properties. This type of lamp is also finding application in sports lighting for HDTV broadcasting, lighting in museums and art galleries, etc. by utilizing its excellent color rendering properties. Metal halide lamps, however, contain mercury as a fill in large quantities amounting to several tens of milligrams per cubic centimeter of content volume, and it is strongly desired to eliminate mercury from the viewpoint of environmental preservation.
Compared with electrode arc discharge lamp systems, electrodeless discharge lamp systems have the advantage that electromagnetic energy can be easily coupled to the fill and it is therefore easy to eliminate mercury from the fill used for light emission by discharge. Furthermore, since there are no electrodes within discharge space, blackening of bulb inner walls due to electrode evaporation does not occur. This significantly improves lamp life.
Non-mercury fills for prior art HID lamps will be described below by way of example. In the electrodeless discharge lamp disclosed in Japanese Patent Unexamined Publication No. 3-152852, xenon is used as a discharge gas, and LiI, NaI, TlI, InI, etc. as luminescent substances are sealed within the lamp, producing white light by combining monochromatic line spectra radiated from these luminescent substances. This prior art discloses as a discharge excitation means a means by inductively coupling RF energy.
In the high power lamp disclosed in Japanese Patent Unexamined Publication No. 6-132018 (U.S. Pat. No. 5,404,076), S.sub.2, Se.sub.2, etc. as luminescent substances are sealed within the lamp, and a greenish white light is produced from the continuous spectrum of molecular radiation. This prior art discloses a discharge excitation means utilizing microwave energy.
Furthermore, U.S. Pat. No. 3,259,777 discloses an invention relating to an electroded metal halide lamp that employs a fill belonging to a metal halide, such as indium iodide used in the present invention. In this prior art, the lamp is operated using electrical energy high enough to heat the electrodes nearly to their melting point in order to cause the metal halide, such as indium iodide, to discharge at high power.
However, the electrodeless discharge lamp disclosed in Japanese Patent Unexamined Publication No. 3-152852 has had the problem that if the proportions of Na and Tl that emit light in regions of high spectral luminous efficiency are increased to increase efficacy, color rendering properties degrades, and if the color rendering properties are to be enhanced, the efficacy has to be decreased. Another problem that has been pointed out is that indium and thallium iodides produce a continuous spectrum at high pressure with a resultant decrease in line spectral causing a color shift. Furthermore, the light characteristics produced by a combination of line spectra, such as disclosed in Japanese Patent Unexamined Publication No. 3-152852, have poor color reproducibility, and it is difficult to obtain satisfactory color rendering properties.
With the high power lamp disclosed in Japanese Patent Unexamined Publication No. 6-132018, even if the kind of gas and the conditions of the fill are changed, chromaticity is always displaced from the black body locus substantially toward green, and it is not possible to obtain a satisfactory white light. A method that can be considered to improve the color characteristics of the high power lamp in Japanese Patent Unexamined Publication No. 6-132018 is to add some kind of metal compound as a luminescent substance and thereby add a line spectrum to change the chromaticity. However, metal sulphides produced by reaction of the added metal compound with sulphur are often relatively stable and low in vapor pressure and are difficult to turn into a plasma. This has lead to the problem that the kinds of metals that can be added are limited, reducing freedom in light color design and making it difficult to improve color rendering properties. Furthermore, when the spectral characteristics of the emission is changed by adding a fill or by using a color temperature conversion filter, spectral emission intensity increases in regions, other than green, where spectral luminous efficiency is low, necessarily resulting in a decrease in efficacy.
In U.S. Pat. No. 3,259,777, on the other hand, for lamp operation with electrodes and with non-mercury fills a considerable load is applied to the electrodes since the lamp is operated near the melting point of the electrodes. With this lamp design, therefore, rapid blackening of bulb inner walls occurs because of electrode evaporation, and a marked drop in lamp life is inevitable.